How much credence is there to the idea that speaking engagements are often a way of getting around campaign finance laws? That could have something to do with it, although it's not as if she needs to announce before spring.
At the rate of 14 delivered or scheduled so far over the past year or so, at $250,000 per speech, that amounts to $3.5 million total - in contrast, in 2007 she was raising north of $20 million per quarter. From a financial standpoint her best bet would be to announce as early as possible so she would have the most fundraising quarters under her belt as possible.
It came out just last week that UCLA paid Clinton $300k to speak - and that was, according to UCLA staff, the "special university rate."